Gorging on Dinosaurs

Fresh vertebrate blood is not exactly everyone's ideal meal. But for a few animals such as leeches, vampire bats, and hematophagous insects, it represents haute cuisine. And even some humans, like the Masai of Kenya, are well known for surviving on a mixture of milk and blood drawn from cows and goats. But by far, the largest group of animals to develop this proclivity is bloodsucking insects, and they have truly perfected this habit because their very survival has depended on this sanguinary diet since the Cretaceous.

Even when we slather ourselves with repellants, bloodsucking insects can make our lives miserable. How could something so small cause so much discomfort? Not only do they annoy us by buzzing around our heads and crawling over our faces, but they inflict wounds that are downright painful and leave swellings that can itch for days. Armed and dangerous, we call them biting insects; however, they really don't bite. While we chomp by placing our teeth around an object and then bringing our jaws together, biting in bloodthirsty insects is much more complicated, and if this sophisticated feeding system didn't result in so much human suffering, we might admire it as a marvel of invention.

They begin with the arduous task of piercing or cutting away a small portion of skin, adding saliva that contains anticoagulant factors, and then finally sucking up the blood. The job is performed with minute needle-like appendages called stylets, and every insect group has their own particular modifications. Most have narrow, elongate mandibles lined with serrated edges much like steak knives. To assist, another pair of slender, ser rated structures called maxillae is often employed. Positioned between this set of cutting instruments is a trough-shaped structure, the hypopharynx, containing a channel through which salivary secretions pass down into the wound. Protecting the mandibles and maxillae from above is the labrum, the underside of which contains a food channel that carries blood from the victim up into the insect. These precision cutting tools resemble delicately designed surgeon's implements.

Each type of bloodsucking insect has its own unique but equally effective method of penetrating vertebrate skin (fig. 22). Mosquitoes are capillary feeders. They first insert their stylets beneath the skin and search for a blood vessel. Once found, the wall of the capillary is punctured, the mouthparts enter, and blood is sucked up directly into the alimentary tract through the food channel. There is usually no fluid left on the surface of the skin when the operation is finished. Blackflies, biting midges, sand flies, and horseflies employ the less-refined pool-feeding method. A hole is cut in the skin, severing several capillaries. Blood is drawn up as it collects in the open wound. Upon completion, there is usually some residue left on the surface of the skin, which often attracts other insects.

The blood meal consists not only of plasma and cells, but any microorganisms that might be developing inside these cells or floating free in the plasma. These pathogens are then passed on to the next victim with the saliva, explaining how biting insects acquire and spread infectious agents.

The main function of the blood meal, which is approximately 20% protein, is to supply nutrients for egg production. It also provides some nourishment for the adults, and mosquitoes have been kept alive for over 80 days on some 30 blood meals.140 Aside from blood, biting flies generally require sugar supplements for long-term survival.141142

These are obtained from various plants (nectar, nectary glands, fruits, plant juices) as well as from insects in the form of honey-dew, and also appear to be important for the development of some pathogens they might carry. Female sand flies obtain sup-

Figure 22. Different feeding methods of bloodsucking insects, and dinosaur scales. A. Capillary or tube feeding by mosquitoes. B. Micro-pool feeding by biting midges. C. Macro-pool feeding by tabanids. Below are scale patterns of dinosaurs and a chameleon. Left, a sauropod. Note the angular edges of the scales and their close fit. Middle, a hadrosaur (Corythosaurus). Here the scales have rounded edges with more exposed skin between them. Right, a mountain chameleon (Chamaelo montium). The scale pattern and scale dimensions are closer to that of the hadrosaur. All drawn to the same scale. Dark areas represent exposed skin surrounding the scales.

Figure 22. Different feeding methods of bloodsucking insects, and dinosaur scales. A. Capillary or tube feeding by mosquitoes. B. Micro-pool feeding by biting midges. C. Macro-pool feeding by tabanids. Below are scale patterns of dinosaurs and a chameleon. Left, a sauropod. Note the angular edges of the scales and their close fit. Middle, a hadrosaur (Corythosaurus). Here the scales have rounded edges with more exposed skin between them. Right, a mountain chameleon (Chamaelo montium). The scale pattern and scale dimensions are closer to that of the hadrosaur. All drawn to the same scale. Dark areas represent exposed skin surrounding the scales.

plementary plant juices by "biting" stems in the same manner they penetrate skin.143

How do hematophagous insects locate and track their victims? Detecting a host can be accomplished visually, but odor also plays an important role, and many compounds in vertebrate skin serve as attractants, especially carbon dioxide. Even small amounts of this gas, such as from an animal's breath, can grab their attention. Heat also is a lure and many biters are able to discern the smallest temperature gradients in the atmosphere.142 Some bloodsuckers are generalists and attack mammals, birds, reptiles, and amphibians. This definitely favors their survival since if one host is unavailable, another is there to replace it. Others are more finicky and target only a few selected animals. How many blood meals does a biting fly take during a lifetime? Most imbibe at least twice, which is just enough to supply nourishment for two egg clusters and also the basic number needed for pathogens to be both acquired and transmitted.142

The various types of biting arthropods that we believe fed on and transmitted pathogens to Cretaceous dinosaurs are discussed in the following pages. Some, which probably gorged on dinosaurs but are not discussed, include snipe flies (Rhagion-idae) and athericid flies (Athericidae), which attack mammals today and existed throughout the Cretaceous. Their modern counterparts are not known to vector any microorganisms. Also extinct Cretaceous chironomids, tanyderids, corethrellids, and mecopterans that had biting mouthparts were also capable of feasting on dinosaurs, but whether they were carrying pathogens is unknown (color plates 10, 11A, 11B, 11C).

Piercing Dinosaur Skin

When one considers the immense size and strength of many dinosaurs like sauropods and tyrannosaurs, it might appear that tiny biting insects would be up against insurmountable odds. Surely such animals had thick, impenetrable skin. But the reality is that except for the back and sides of plated stegosaurs and armored ankylosaurs, dinosaur skin was surprisingly thin and reptilian, in fact very similar to that found on present day chameleons and Gila monsters (fig. 22). How amazing that creatures 40 or even 100 feet long and weighing many tons had the same size and type of scales as these small lizards!

The dry skin of reptiles is covered with scales, which are localized thickenings of the horny material keratin situated in the stratum corneum of the epidermis. They usually form a continuous layer and vary in size, shape, and texture. Imbricate scales overlap but scales of some lizards are juxtaposed to one another with exposed bare areas of unprotected skin between them. Scales can be small and polygonal, large and plate-like, and smooth or possess various ridges or keels. Size may be directly related to flexibility, with small ones located on mobile areas and larger ones on immobile areas such as the top of the head or back. Certain types of reptiles have tuberculate scales that are modified into wart-like raised extensions or tubercles. Scales are periodically shed with the skin as an entire sheet or in pieces of various sizes. Sometimes individual scales are discarded separately. This process occurs more frequently in growing stages, and the intervals between shedding vary between species. Whether dinosaurs molted periodically is unknown, but given their skin covering, highly likely.

Only a very few fossils of dinosaur skin have been found, and over half of these were from hadrosaurs, which were covered with tuberculate scales that fell into three categories.145 The first consisted of uniformly small, ground tubercles with no definite arrangement. The second type was flattened, polygonal-shaped pavement tubercles ranging from 0.19 to nearly 0.25 of an inch in diameter, with smooth surfaces and mostly uniform margins. The third type resembled very large radially sculptured, limpetlike cones reaching 1.26 to 1.50 inches in diameter and 0.30 of an inch in height. One hadrosaur from the Late Cretaceous of Canada, Edmontosaurus, had extremely thin skin covered with ground and pavement tubercles.145 Another duckbill, Anatosau-rus, had ground and pavement types with larger scales on the sides, back, and tail and smaller ones on the shoulders and inner thighs.148 Finally, the skin of Corythosaurus was composed of flat, polygonal tuberculate scales.147 Many hadrosaurs had a skin fold or frill adorned with variously sized tubercles extending along their backbones.

Examples of ceratopsian skin have also been recovered. The integument of the horned dinosaur Chasmosaurus, also a Late Cretaceous specimen, was composed of polygonal, non-overlapping scales.149 Another ceratopsian, Centrosaurus, had a similar integumental pattern with large tuberculate plates surrounded by smaller ones. Relatively thin skin with tuberculate scales was noted on two specimens of the ornithopod dinosaur Iguanodon from the Early Cretaceous.146150 In one, the skin was covered with scales ranging from 0.14 to 0.23 inches in diameter, and in certain areas they ran together and the skin appeared almost smooth. The outer surface of the other was composed of very thin skin dotted with small convex tubercles varying from 0.13 to 0.24 inches in diameter. In some areas, these were larger and flatter, while in others, the tubercles appeared to coalesce and were almost imperceptible.150

Sauropods such as Camarasaurus had epidermis with larger hexagonal, non-overlapping convex plates ranging from 0.43 to 1.10 inches in diameter. The plates in this tuberculate integument became smaller and rounded towards the armpits. The skin of another sauropod was also relatively thin and the scales formed a rosette pattern. These were 0.80 to 1.20 inches in width and covered with minute papillae. On portions of the skin were dermal spines similar to those on the backs of iguanas and along the tails of crocodilians.151

Pebbly skin impressions of the tyrannosaurs Gorgosaurus and Daspletosaurus show that the common ground plan of tuberculate scales ran through the entire dinosaur line. The armored dinosaurs, ankylosaurs and nodosaurs, along with the titanosaur sauropods, bore dorsal and/or lateral bony plates, also known as body ossicles, osteoderms, or scutes, similar to those found on turtles.

What was the skin surface in feathered dinosaurs? The genera Sinosauropteryx, Protoarchaeopteryx, Caudipteryx, and Beipiaosau-rus from the Cretaceous of China are all described as having feathers on part or all portions of their bodies, including their tails, and some even consider these fossils as birds. While a few paleontologists believe that the protrusions covering these fossils are a kind of quill-like extension of the scales, others feel that there is no doubt about them being true feathers.152 Although the actual integument of these dinosaurs was not preserved, in present day birds, feathers arise from discrete areas surrounded by membranous skin. All groups of biting arthropods discussed in this book feed on birds, so the presence of feathers on dinosaurs would not deter the bloodsuckers.

So where would these arthropods feed? At first glance, it would appear that obtaining blood through the scaly skin of dinosaurs was an impossible feat. However, there were several ways available. If the scales were thin, arthropods had no problem penetrating directly through them. When too thick, the biter might have utilized minute openings in the scales similar to those occurring on some extant reptiles.144 While only a few microns wide, the holes in all likelihood were enlarged with the cutting edges of the mandibles or maxillae. However, if these openings did not occur on dinosaur scales, arthropods were capable of feeding on the softer, non-keratinized skin between the scales. The thin integument exposed around the tubercles was the Achilles' heel of dinosaurs and definitely offered no protection against bloodsucking arthropods. These biters could simply have fed around the margins of the tubercles anywhere on dinosaurs and did not need to search out specific, less-keratinized locations such as around the mouth, eyes, ears, or nasal openings, although they also were prime sites. The loose skin lobes on the forelimbs of some hadrosaurs also offered choice feeding areas. Even the dermal plates of Stegosaurus, which were lined with blood vessels on their outer surfaces, undoubtedly provided feeding places.30 And if dinosaurs periodically shed their skin, they became particularly vulnerable to biting arthropods at those times.

The similarity in size, shape, and patterns between the scales of dinosaurs and chameleons is striking154 (fig. 22). Since chameleons are bitten by mosquitoes, ticks, and sand flies, all of which introduce pathogens into these colorful lizards, the dinosaurs certainly were sitting ducks for bloodsuckers too. They probably had few means of avoiding the pests, unless the small forelimbs on some could have been employed to shoo away flies or scrape off ticks. Some relief may have been found in water, and perhaps the large sauropods, and even hadrosaurs, sub merged their under parts as much as possible for protection from the voracious biters.

The wide host range of most present-day bloodsucking arthropods indicates that it really wasn't significant if dinosaurs were warm- or cold-blooded155 or if they were covered with tubercu-late scales or feathers. These formidable micro-predators still would have been able to locate feeding sites on Cretaceous dinosaurs just as they are able to use birds, mammals, reptiles, and amphibians as a smorgasbord of protein delights today.